This project will study patients with Parkinson's disease that is not satisfactorily controlled by medications. Patients will undergo implantation of electrodes for deep brain stimulation of the globus pallidus or subthalamic nucleus, an experimental treatment for Parkinson's disease. The purpose of this study is to determine how deep brain stimulation produces its beneficial effects, how those effects compare to those of levodopa and how deep brain stimulation affects the response to levodopa. The duration of this study is 3 years with the first evaluations 3 months before the surgical procedure and the final evaluations 2 to 3 years after surgery.